This guide explains when to plant basil in the UK, why it struggles so often in British conditions, and how to grow it successfully whether you are starting from seed or working with a bought plant. For the full growing process from seed to harvest, see How to Grow Basil from Seed in the UK.

Quick Answer

In most UK gardens, basil is sown indoors from late March to May and moved outside only from June onwards once temperatures are reliably warm. Basil is extremely cold-sensitive — even a cool night can set it back significantly. It grows best as an indoor or sheltered outdoor herb through most of the UK summer.

Why Basil Is Difficult to Grow in the UK

Basil originates from tropical climates where temperatures are consistently warm and humidity is high. The UK offers almost the opposite — cool nights, unpredictable summers, and damp conditions that basil finds stressful.

This is why supermarket basil plants fail so often. They are grown in controlled glasshouse conditions and sold before the weather is warm enough to support them. Placed on a cold windowsill or outside too early, they deteriorate quickly.

Growing basil successfully in the UK means working with its needs rather than against them — starting it at the right time, keeping it warm, and understanding that it will always perform better in a warm, sheltered spot than in open garden beds. If you are new to growing herbs and vegetables, gardening for beginners in the UK covers the foundations that make growing easier across the board.

When to Sow Basil Seeds Indoors

Basil can be sown indoors from late March through to May. This is the most reliable way to grow it in the UK because it gives seeds the warm, stable conditions they need to germinate and establish properly before outdoor temperatures are consistently warm enough.

Basil seeds need warmth to germinate well — ideally above 18°C. A warm windowsill, propagator or greenhouse shelf all work. In cooler rooms germination is slow and uneven, and seedlings that do emerge often stay weak.

Sow seeds thinly into small pots or modules filled with good quality compost. Best compost for vegetables in the UK is worth reading if you are unsure which compost to use — the same principles apply for herbs. Cover seeds lightly and keep the compost moist but not waterlogged. Seedlings usually appear within one to two weeks in warm conditions.

Once seedlings have two pairs of leaves they can be thinned or potted on. Basil benefits from being grown in small groups rather than as single plants — this helps with bushier growth and keeps individual plants from becoming leggy.

When to Move Basil Outside

Basil should only be moved outside once night temperatures are consistently above 10°C — in most UK gardens this means June at the earliest. Even in a warm May, a cold night can cause leaves to blacken and plants to collapse very quickly.

Before moving basil outside it needs to be hardened off, just like any tender plant grown indoors. Basil is particularly sensitive to the transition between warm indoor air and cooler outdoor conditions. Read how to harden off plants in the UK before putting basil outside — a week of gradual acclimatisation makes a significant difference to how well it settles.

Even after hardening off, basil performs best in a warm, sheltered outdoor spot — against a south-facing wall, in a sheltered corner, or in a container that can be brought inside if a cold night is forecast. It is not a plant for open, exposed garden beds in the UK.

Checking UK last frost dates by postcode before moving basil outside is a sensible step — basil is one of the most frost-sensitive plants you are likely to grow.

Month-by-Month Guide to Basil

March

Late March is the earliest sensible time to sow basil indoors in most UK gardens. Conditions need to be genuinely warm — a cold spare room will not give good results. A propagator or warm south-facing windowsill is ideal at this time of year.

April

April is a good month for indoor sowing. Basil started in April will be well established by the time outdoor conditions are warm enough in June. Keep plants on the warmest windowsill available and away from cold draughts.

May

May is the last good month for sowing basil if you want plants ready for a full summer of use. Basil can still be sown in May and will establish quickly in warm conditions. Do not move plants outside yet — even warm May days can be followed by cold nights that damage basil badly. This is also the month to start hardening off April sowings ready for outdoor planting in June. If you are growing other tender crops this month too, what to sow in May in the UK and what to plant in May in the UK cover everything that can go outside this month.

June

June is when hardened-off basil can go outside in most UK gardens. Choose the warmest, most sheltered spot available. A container on a south-facing patio or against a warm wall works much better than an open bed. Keep an eye on the forecast and bring plants in if a cold night is expected.

July and August

The best growing months for basil in the UK. Plants established by now will grow strongly in warm weather and produce well if picked regularly. This is also the time to prevent flowering — pinching out flower heads as they appear keeps plants producing fresh leaves for longer.

September

Temperatures begin to drop in September and basil will start to deteriorate outdoors. Bring container plants inside before the first cold nights arrive. Plants left outside too long in autumn rarely recover. September is also a good time to take cuttings from healthy plants to overwinter on a warm windowsill indoors.

Where to Grow Basil in the UK

The best place to grow basil in most UK gardens is on a warm windowsill indoors or in a sheltered outdoor spot that gets full sun and is protected from cold wind. A south-facing windowsill in the kitchen is the classic choice and works very well because the warmth is consistent and picking is easy.

Outdoors, the ideal spot is against a south-facing wall or fence where the plant gets reflected warmth as well as direct sun. A patio or courtyard that traps heat works well. Open vegetable beds in exposed positions rarely give good results with basil in the UK — the combination of wind and cool nights is too much for it.

Containers work particularly well for basil because they can be moved — brought inside when cold nights are forecast and positioned to follow the sun through the day. If you are growing other vegetables in pots this summer, can you grow vegetables in pots in the UK covers how to manage containers through the season.

What Soil and Compost Basil Needs

Basil grows best in fertile, well-drained compost or soil. It does not like sitting in wet or waterlogged conditions — roots in cold, damp compost deteriorate quickly and the plant collapses from the base up.

For container growing, a good quality multipurpose compost works well. Make sure pots have adequate drainage holes and do not sit in standing water. Peat-free compost in the UK covers what to look for and what to avoid if you are choosing compost for containers this season.

In the ground, basil prefers light, open soil that warms quickly. If your soil is heavy or tends to stay wet after rain, growing basil in containers will always give better results than planting it directly into the ground. Why garden soil stays wet explains why some soils hold water for so long and what can be done about it.

How to Keep Basil Producing Through Summer

Regular picking is the single most important thing you can do to keep basil productive. Picking encourages bushy growth and delays flowering. Always pick from the top of each stem, removing the growing tip and leaving two pairs of leaves below. This causes the plant to branch and produce more stems rather than growing tall and straggly.

Once basil flowers, leaf production slows significantly and the flavour of remaining leaves diminishes. Pinch out any flower buds as soon as they appear — do not wait until they open. If plants do flower and you want to extend their useful life, remove the flower heads completely and the plant will often recover and put on fresh growth.

Watering needs some care. Basil needs consistent moisture but hates waterlogging. Water at the base of the plant rather than over the leaves, particularly indoors where wet foliage in low light leads to mould. Allow the surface of the compost to dry slightly between waterings. How often to water plants in the UK covers the principles that apply to herbs and vegetables alike.

Light feeding every two to three weeks with a balanced liquid feed helps keep plants productive through summer. Avoid overfeeding — too much nitrogen produces lush, soft leaves with less flavour. Feeding vegetables properly in the UK explains why restraint with feeding almost always produces better results than excess.

What to Do With Supermarket Basil Plants

Supermarket basil pots are usually packed with many seedlings grown very close together to look full and lush on the shelf. They are not intended to last — they are sold as a short-term product rather than a plant.

To get more from a supermarket basil plant, split it into three or four smaller clumps, pot each one into its own container with fresh compost, and keep them on a warm windowsill. Given a little space and proper conditions, each clump will often recover and produce well for months.

Do not put supermarket basil outside until June at the earliest. These plants have been grown in warm glasshouses and are even more sensitive to cold than garden-grown plants. Moving them outside too soon is the most common reason they fail so quickly.

Common Mistakes With Basil in the UK

Moving basil outside too early is the most common mistake by a significant margin. Even experienced gardeners underestimate how quickly basil reacts to cold nights. June is the safe month for outdoor planting across most of the UK — not May, regardless of how warm the days feel.

Overwatering is the second most common problem. Basil roots in cold, wet compost deteriorate quickly and the plant collapses in a way that looks like it needs more water, when in fact water is the problem. If basil wilts despite moist soil, the issue is usually root stress rather than drought. Why vegetable plants wilt in UK gardens explains this pattern in more detail.

Placing basil on a cold or draughty windowsill — even indoors — slows growth and weakens plants. North-facing windowsills, spots near doors, or positions where cold air comes in around window frames are all problematic. The warmest, brightest spot you have is always the right choice for basil.

Letting plants flower without pinching them out is another common mistake. Once basil runs to flower the plant puts its energy into seed production rather than leaf growth, and the flavour and quantity of leaves drops off quickly. For a broader look at what trips up beginners, common gardening mistakes beginners make is a useful read.

Basil Alongside Other Summer Crops

Basil is a natural companion to tomatoes in the kitchen and in the garden. It thrives in the same warm conditions that tomatoes need and is traditionally grown alongside them. If you are growing tomatoes this summer, the timing for both crops is similar — indoors now, outside in June once conditions are genuinely warm.

It also sits well alongside other warm-season crops like courgettes, cucumbers, sweetcorn and French beans — all of which share similar timing requirements and do best once the UK summer is properly underway. For the full picture of what to grow and when, the the vegetable sowing calendar and when to plant vegetables in the UK cover the whole season in one place.

Frequently Asked Questions

A Sensible Place to Start

Basil is not a difficult herb to grow in the UK once you stop fighting its needs and start working with them. Sow indoors in April or May, keep it warm, harden it off carefully, and do not rush it outside before June.

Give it your warmest spot, pick it regularly, and keep the flowers pinched out. Do those things consistently and basil will produce well from June right through to September — far better than anything you will ever find on a supermarket shelf.

If you are building out a full summer growing plan alongside basil, May gardening jobs in the UK covers everything that needs doing right now, and easy vegetables to grow in the UK is a good guide if you are deciding what else to grow this season.

The other thing that makes May useful is variety. You can sow fast crops like salad leaves and radishes that will be ready in weeks. You can sow slower crops like carrots and beetroot that will fill the veg patch through summer and autumn. You can start leeks and brassicas that won’t be harvested until winter. And you can get flowers in the ground that will be blooming by July. All in the same month, in the same beds, without a greenhouse in sight.

This guide covers what to sow in May in the UK in detail, crop by crop, so you know exactly what goes in the ground now, what to start indoors, and what to avoid rushing before conditions are ready. For a broader look at everything you can do in the garden this month beyond just sowing, May Gardening Jobs in the UK covers the full picture.

Getting the Soil Ready Before You Sow

Before anything goes in the ground it’s worth spending ten minutes on the soil. You don’t need to dig everything over or add bags of compost before every sowing, but a bit of basic preparation makes a real difference to how well seeds germinate.

The main thing you want for direct sowing is a fine, crumbly surface that seeds can make good contact with. If your soil is lumpy, compacted, or has a crust from recent rain, break it up with a rake and work it into a reasonable tilth before sowing. Seeds that sit in air pockets or can’t contact moist soil consistently will germinate patchily or not at all.

If a bed has had nothing added to it for a while, working in some compost before sowing is worthwhile. It doesn’t need to be a deep dig, just mixing some into the top layer where the seeds will sit. How to Improve Garden Soil in the UK covers the practical side of this, and Best Compost for Vegetables in the UK covers what’s actually worth buying.

One more thing: check moisture levels before sowing. The soil should be damp but not wet. If it’s dry, water it a day before sowing rather than after, so the moisture is evenly distributed through the surface layer when the seeds go in.

Vegetables to Sow Directly Outside in May

Direct sowing means putting seeds straight into the ground outdoors rather than starting them in pots or trays. May is the best month of the year for this because soil temperatures are high enough for seeds to germinate quickly without rotting, and the weather is generally warm enough to keep growth moving.

Carrots

Carrots are one of the most rewarding things to sow in May. Germination is far more reliable now than in March or April when the soil is still cold and wet, and you get much less of the patchy, uneven emergence that frustrates early season sowing.

Sow thinly into drills about 1cm deep and 30cm apart. Carrots don’t like transplanting so they need to go straight into their final position. Thin to about 5-8cm apart once they’re established, or you’ll get lots of small, twisted roots fighting for space.

One of the best things you can do with carrots is sow in succession, a short row every two to three weeks through May and into June, rather than sowing a large amount all at once. You end up with harvests spread over months rather than everything coming ready at the same time. When to Plant Carrots in the UK covers the timing in full.

Beetroot

Beetroot is probably the most beginner-friendly crop you can sow in May. It germinates quickly in warm soil, grows without drama, tolerates a certain amount of neglect, and produces harvests you can actually use in the kitchen. If you’re not sure where to start in May, beetroot is always a safe choice.

Sow about 2cm deep in rows 30cm apart. Each beetroot “seed” is actually a cluster of seeds, so you’ll almost always get more than one seedling per planting position. Thin down to one plant per position once they’re a few centimetres tall. Like carrots, succession sowing every two to three weeks gives you a much more manageable harvest. When to Plant Beetroot in the UK has the full timing guide.

Peas

Early May is still within the pea sowing window, but it is the tail end of it. Peas prefer cooler growing conditions and once summer heat arrives they struggle, so anything sown after mid-May is a bit of a gamble depending on your location. Get them in during the first two weeks of May for the best results.

Sow about 5cm deep in double rows, with seeds roughly 8cm apart within the row. Peas need support as they grow, so put canes or pea sticks in at sowing time rather than waiting until they’re already flopping over. When to Plant Peas in the UK explains the window clearly and what to expect from later sowings.

French Beans

French beans are a warm-soil crop and mid-May is the earliest you should sow them outdoors. Any earlier and the seeds tend to sit in the ground without germinating, or they rot before they get the chance. The difference between sowing French beans in late April and mid-May is often the difference between success and a failed sowing.

Sow about 5cm deep in rows 45cm apart, with seeds roughly 15cm apart in the row. They grow quickly once conditions are right and you can expect to be harvesting within 10-12 weeks of sowing. Climbing varieties need support; dwarf varieties are more compact and don’t need staking. When to Plant French Beans in the UK covers the timing in detail.

Runner Beans

Runner beans follow the same logic as French beans, mid to late May for outdoor sowing, once the soil is warm and overnight temperatures have settled above 10°C. They’re one of the most productive crops in a UK vegetable garden, and a couple of plants will produce more beans than most families can keep up with through summer.

Sow 5cm deep, about 15-20cm apart along a row of canes or a wigwam structure. Put the supports in before or at the same time as sowing, not after, so you’re not disturbing roots later. Water well after sowing and keep an eye on soil moisture in the first few weeks. When to Plant Runner Beans in the UK has the full timing guide.

Salad Leaves and Lettuce

Salad leaves, cut-and-come-again mixes, spinach, and lettuce are among the fastest and most satisfying crops to sow in May. Germination is quick, harvests can start within four to six weeks of sowing, and you can cut the leaves repeatedly rather than pulling the whole plant.

Scatter seeds thinly over a prepared bed or sow in shallow drills about 1cm deep. Thin once seedlings are a few centimetres tall. Like carrots and beetroot, succession sowing a small amount every couple of weeks makes much more sense than sowing a large amount at once and ending up with more salad than you can eat before it bolts.

Spinach is worth growing separately from salad mixes as it has slightly different needs, it prefers not to dry out and can bolt quickly in hot, dry conditions. When to Plant Spinach in the UK covers the timing and how to manage it through summer. For lettuce specifically, When to Plant Lettuce in the UK explains the best varieties for May sowings.

Spring Onions and Radishes

Both of these are worth sowing if you want something genuinely quick. Radishes can be ready in as little as four weeks from sowing in May, making them one of the fastest crops in the kitchen garden. Spring onions take a little longer, around eight weeks, but they’re dependable and can be used fresh from the ground.

Sow radishes thinly in shallow drills and thin to about 3cm apart once germinated. Don’t leave them in the ground too long once they’re ready, they turn woody and unpleasant within days of reaching harvest size. Sow a small amount every ten days for a continuous supply rather than a single large sowing.

Courgettes

Courgettes can be sown directly outside from late May in sheltered, mild gardens. Most people start them indoors in April for planting out in May, but if you haven’t done that, a direct outdoor sowing in the last week of May is perfectly viable in most UK gardens. They germinate fast once soil temperatures are consistently above 15°C.

Sow two seeds per position, about 2cm deep, and remove the weaker of the two seedlings once they’re established. Give each plant plenty of room, they spread considerably. How to Grow Courgettes in the UK covers the full process from sowing to harvest.

Sweetcorn

Late May is the right time to sow sweetcorn directly outside in warmer parts of the UK. In the north or on exposed sites it’s safer to start indoors in late April or early May and plant out in June. Sweetcorn is wind-pollinated and needs to be grown in a block, not a single row, so each plant pollinates its neighbours.

Sow 2.5cm deep in a grid pattern with about 45cm between plants. They need a warm, sheltered spot and consistent moisture to crop well. Don’t bother growing sweetcorn in a cold or shaded garden, it just doesn’t produce enough in poor conditions to be worthwhile. How to Grow Sweetcorn in the UK covers the full process from seed to harvest.

Potatoes

Maincrop potatoes can still go in during early May in most UK gardens, particularly in northern areas where later planting is normal. If you’re in the south and haven’t planted yet, early May is getting late for maincrops but still worth doing. First and second earlies planted now will produce a later harvest than usual but will still crop. When to Plant Potatoes in the UK covers the full timeline by variety and region.

Flower Seeds to Sow in May in the UK

May is an excellent month for sowing flowers directly outside. Hardy annuals in particular do well when sown now, they establish quickly in warm soil and most will be flowering by July or August. For the full guide including bedding plants and summer bulbs, What Flowers to Plant in May in the UK goes much deeper into flowers specifically.

Nasturtiums

Nasturtiums are almost impossible to fail with in May. Sow them where you want them to flower, cover lightly with soil, and water in. They germinate within a week in warm conditions and produce flowers from June onwards. They work well in vegetable gardens too, attracting aphids away from other crops and bringing in beneficial insects.

Cornflowers

Cornflowers sown directly in May will flower from July and keep going until the first frosts. Sow thinly in drills, thin to about 15cm apart, and deadhead regularly to keep them producing. They’re fully hardy and ask for very little once established.

Calendula

Calendula (pot marigold) is another May sowing that gives a lot back for very little effort. Sow directly outside, thin to 30cm apart, and deadhead regularly. Like nasturtiums, they work well alongside vegetables and are edible, the petals can be used in salads.

Sunflowers

Sunflowers can be sown directly outside in May once frost risk has passed. Sow 2.5cm deep in their final position as they don’t transplant well. Give them full sun and shelter from wind if possible. They’re one of the most satisfying plants to grow from seed and children in particular love watching them develop.

Borage, Phacelia, and Annual Poppies

All three of these can be scattered directly where you want them to flower in May. They germinate readily in warm soil and are excellent for attracting pollinators. Borage in particular is worth growing near vegetable beds as it draws bumblebees and other beneficial insects.

What to Sow Indoors or Under Cover in May

Not everything sown in May goes outside immediately. A few crops benefit from being started indoors or under cover in May for planting out later in the season.

Leeks

Leeks are a long-season crop that need starting in May to give them enough time to develop before autumn. Sow thinly in a seed tray or small pot indoors and they’ll be ready to plant out into their final positions in June or July. They’re one of the most dependable winter vegetables and well worth the growing time. When to Plant Leeks in the UK covers the full sowing and planting schedule.

Kale and Purple Sprouting Broccoli

Kale and purple sprouting broccoli both need to be started in May if you want harvests through winter and early spring. Sow in individual pots or a seed tray under cover and plant out in June once they’ve developed a few true leaves. These are tough, reliable crops that occupy space in the garden for a long time, so it’s worth getting the timing and spacing right before committing. When to Plant Brassicas in the UK covers the full schedule for all brassica types.

Brussels Sprouts

If you want Brussels sprouts for Christmas, May is genuinely your last chance to sow them. They need a very long growing season, around six months from sowing to harvest, so seeds going in now will produce sprouts from November onwards. Start indoors or under cover and plant out into a firm seedbed in June.

Basil

Basil needs warmth to germinate well and is best sown indoors in May rather than outside. Full timing guidance is in When to Plant Basil in the UK, and How to Grow Basil from Seed in the UK covers the full process from sowing through to harvest. It’s one of the most temperature-sensitive herbs and even a mild cold snap outdoors can check its growth badly. For the wider picture across all UK herbs, see growing herbs at home. Sow in small pots on a warm windowsill, keep it there until June, and only move it outside once you’re sure overnight temperatures have settled.

Succession Sowing in May

One of the most useful habits to get into with May sowing is thinking in terms of succession rather than single large sowings. The idea is simple: instead of sowing a whole row of carrots or a large patch of salad in one go, you sow a small amount and then sow another small amount two to three weeks later, and so on through the growing season.

The result is that you get a steady supply of crops coming ready at different times rather than everything being ready at once. This is particularly useful for fast-growing crops like radishes, salad leaves, spring onions, and lettuce, where a single large sowing can produce more than you can use before it goes past its best.

For slower crops like carrots and beetroot, succession sowing spreads the harvest across a longer period and also means that if one sowing is hit by pests or bad weather, you haven’t lost everything.

Common Sowing Mistakes in May

A few mistakes come up repeatedly in May that are worth knowing about before you start.

Sowing too deep. Most vegetable seeds are smaller than you think and don’t need to go in very deep. The general rule is to sow at a depth of roughly twice the diameter of the seed. Sowing too deep means seeds have to push through too much soil before they reach light, and many don’t make it. If in doubt, sow shallower rather than deeper.

Sowing too thickly. It’s tempting to sow generously to make sure you get enough plants, but overcrowded seedlings are harder to thin, compete badly with each other from the start, and are more vulnerable to fungal problems caused by poor air circulation. Sow thinly and thin properly once they’re established.

Not watering after sowing. Seeds need consistent moisture to germinate. A dry spell right after sowing can stop germination altogether, or worse, start it and then kill the seedlings before they reach the surface. Water the bed gently after sowing and keep the soil evenly moist until germination is underway. How Often to Water Plants in the UK covers the detail on watering through the growing season.

Forgetting to thin. Thinning is the job most beginners skip or put off, and it shows in the final harvest. Crowded seedlings produce small, misshapen crops and are more prone to disease. Thin properly to the recommended spacing and resist the urge to leave them all in hoping they’ll find their own space. They won’t. When to Thin Vegetable Seedlings in the UK explains how to do it without second-guessing yourself.

Sowing warm-season crops too early in May. French beans, runner beans, and courgettes all need warm soil to germinate properly. Sowing them in early May when soil hasn’t fully warmed often results in failed germination or rotting seeds. Wait until mid to late May for these crops and the success rate improves significantly.

Protecting May Sowings from Pests

Two pests are worth thinking about whenever you sow in May.

Slugs are at their worst in warm, damp conditions and young seedlings emerging in May are exactly what they’re looking for. A row of carrot seedlings can disappear overnight if slugs are active in your garden. Check after dark to get a sense of numbers and deal with them before they establish. How to Get Rid of Slugs in the Garden covers the methods that actually make a difference.

Birds can also be a problem with freshly sown beds, particularly for peas and beans where they’ll pull seeds straight out of the ground. A simple covering of netting or a few twiggy sticks pushed into the soil around the sowing area is usually enough to deter them while seeds germinate and establish.

FAQs

A Sensible Place to Start

May gives you more sowing options than any other month in the UK gardening year. Hardy crops like carrots, beetroot, and salad leaves can go straight in the ground right now. Slightly more demanding crops like French beans and runner beans can follow from mid to late May once the soil has properly warmed. Flowers like nasturtiums and cornflowers can go in whenever you’re ready. And slower crops like leeks and brassicas are worth getting started indoors this month for harvests later in the year.

The thing to avoid is trying to do everything at once in the first week of May. Stagger your sowings, do a little at a time, and build in succession sowings for the faster crops. It takes about the same amount of effort as doing it all in one go, and the results over the next four to five months are considerably better.

If you’re not sure where to begin, start with three crops: beetroot, salad leaves, and nasturtiums. All three are low risk, quick to germinate, and immediately rewarding. Once those are in the ground and growing the rest of the season tends to take shape naturally. For everything else worth doing in the garden beyond sowing this month, May Gardening Jobs in the UK covers the full picture. And for the full year of sowing and harvest dates on one page, bookmark the our month-by-month sowing guide.